The present invention relates to brightness control technology on a scanning electron microscope having a function of automatically executing shape measurement of a pattern within an image, and more particularly relates to brightness control technology suitable for a focused ion beam device provided with a function of automatically executing shape measurement of a pattern formed in cross section by cross section machining.
A focused ion beam device such as that shown in FIG. 4 is used when observing the internal structure of samples such as the internal structure of a semiconductor device, and after performing slicing to expose a cross section the cross section is observed as a scanning microscope image. Specifically, since focused ion beam devices can be used for microscopic observation of material processing, in recent years they have often been used in carrying out cross sectional machining and observation of samples. The cross sectional size is generally about a few μm to a few tens of μm. Ions from an ion source 1 are formed into a beam shape by an ion optical system 3, and the ion beam 2 is received through operation of a deflector 4 and irradiated to specified locations on a sample 9. This focused ion beam device provides a scanning ion microscope function, a sputter etching function and a maskless deposition function. Charged particles called secondary ions or secondary electrons ejected by the sample due to irradiation of an ion beam scanned by the deflector 4 are detected by a secondary charged particle detector 6, which functions as a scanning ion microscope by making this detection value correspond to an irradiation position and producing a corresponding image. A sputtering function is achieved by causing sputtering and dispersal of sample material at portions where an ion beam positioned by the deflector 4 is irradiated. In addition, a deposition function is also realized for irradiating a focused ion beam 2 while forming a layer of the material while spraying source material gas from a gas gun 6 to cause formation of a layer of the material at the irradiation position. In a basic slicing sequence using this type of focused ion beam device, as shown in FIGS. 3A-3B, locations within a sample where the cross section is to be observed are first specified.
In order to observe the cross section, it is necessary to make a wide opening in front of the cross section part (the section enclosed by the dotted line). This is the reason that in obtaining a microscopic image of the cross section it becomes necessary to beam scan from a steep angle with respect to the cross section. If a processing region is specified, coarse processing is carried out using an ion beam from a direction vertical with respect to the sample surface, as shown in FIG. 3B. Since this processing is more to machine a hole required for observation than to expose an observation cross section V, rough sputtering using a heavy current beam results, and the observation cross-section V is damaged as shown in FIG. 3B. This sputtering operation can be accomplished with high etching efficiency by spraying assist gas from the gas gun 6 and causing evaporation of the sputtered material using the assist gas. The observation cross section is polished by carrying out etching with a reduced beam current after drilling has been completed, and a neat cross section as shown in FIG. 3C is then exposed. A sample platform after this pretreatment is tilted and beam scanning is performed so as to irradiate an ion beam from a steep angle with respect to the cross section, to obtain a scanning ion microscope image of the cross section to be observed.
However, a display 11 for showing the microscopic image is incorporated into this type of focused ion beam device, and in order to make it easy to view the microscopic image a function for automatically adjusting brightness so as to make brightness information for an overall image a specified average brightness level in the observation field of view is provided in a computer 10. With this function, it is easy to see the entire image on the display, but in the event that an evaluation is made regarding the shape of a pattern formed on the cross section to be observed, there are problems that it is not possible to make out features of a noted pattern because the image at parts of the cross section where the noted pattern exists is either too dim or too bright. This phenomenon is more noticeable when the occupancy rate of the cross section in the observational field of view is low. However, in the case of this cross sectional observation, as described previously, first of all an ion beam 2 is irradiated from above the sample and pretreatment to drill a hole in front of the cross section to be observed is required, and a cross section image is obtained by tilting a sample stage 7 after the slicing processing to scan the ion beam across the sample from a steep angle. Movement of this sample stage 7 is a main cause of field of view slippage when using a microscope at high degrees of magnification. The microscopic image at the time of tilt drive is acquired at a comparatively low magnification factor. The occupancy rate in the observational field of view of the sectioned part constituting a subject of observation becomes low in that case, and a phenomenon occurs whereby it is not possible to observe a noted pattern without appropriate brightness control of the subject image. If it is not possible to observe a pattern, a noted pattern will be perceived in the center of the microscope image while confirming position, making it impossible to perform shape measurement at a high magnification factor.
Also, in slicing of a sample using a conventional device and shape measurement of the slice section pattern, a sequence of operations from slicing before observation to conformation and position adjustment of a noted pattern and shape measurement of the noted pattern must be carried out one at a time by an operator with the focused ion beam device each time, even for a fixed task, which poses a problem with regards to working efficiency. In particular, with tasks for a sample having a repeating pattern structure, there is a lot of repeating of exactly the same tasks, which is stressful to an operator.